Toward Mechanism-Resolved Creep Damage Modelling: A Five-Axiom Constitutive-Theory Charter

Creep damage constitutive models remain essential for assessing high-temperature components; however, structurally credible predictive capability under long-term weld-service conditions has not yet been achieved. Existing formulations reproduce selected uniaxial creep curves, but often fail under low-stress regimes, multiaxial stress states, weld-zone heterogeneity, and extrapolation beyond calibration. This paper argues that these limitations are structural rather than merely parametric and, to the authors’ knowledge, is the first to formulate structural credibility for long-term creep damage prediction as an explicit constitutive-theory admissibility problem. It proposes a preliminary five-axiom charter for mechanism-resolved creep damage modelling. The charter supplements, rather than replaces, classical rational-continuum requirements by adding creep-specific admissibility conditions: deformation and lifetime consistency, mechanism-based formulation, regime integration, scale compatibility, and traceability. The axioms are first formulated at the material-point or homogeneous-material level, where they provide criteria for assessing predictive admissibility with respect to stress level, stress state including multiaxiality, temperature, time, damage mechanism, scale transition, and traceability; weld-zone heterogeneity is then treated as an additional multi-zone application. The charter thereby extends classical constitutive admissibility from mechanical and thermodynamic consistency to predictive admissibility for long-term creep damage. It therefore defines an additional level of constitutive admissibility: predictive admissibility for long-term creep damage, beyond classical mechanical and thermodynamic admissibility. It is grounded in earlier validation questions, high-Cr steel analyses, and evidence on Type IV damage, cavitation, localisation, multiaxiality, and microstructural degradation. Representative time–temperature–stress equations and compact TTS and mesoscopic evidence illustrate possible implementation pathways. TTS is positioned as an implementation pathway, not a closed solution, for moving from curve fitting toward mechanism-resolved predictive modelling.